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Message-ID: <CAA1CXcAOO8KkdF-beDAvZMtEy9K5yT5B3ykxZoL9d01+XSwxPg@mail.gmail.com>
Date: Mon, 17 Feb 2025 14:22:27 -0700
From: Nico Pache <npache@...hat.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
linux-mm@...ck.org, ryan.roberts@....com, anshuman.khandual@....com,
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baohua@...nel.org, jack@...e.cz, srivatsa@...il.mit.edu,
haowenchao22@...il.com, hughd@...gle.com, aneesh.kumar@...nel.org,
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rostedt@...dmis.org, mathieu.desnoyers@...icios.com, tiwai@...e.de
Subject: Re: [RFC v2 7/9] khugepaged: add mTHP support
On Mon, Feb 17, 2025 at 1:55 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 11/02/2025 00:30, Nico Pache wrote:
> > Introduce the ability for khugepaged to collapse to different mTHP sizes.
> > While scanning a PMD range for potential collapse candidates, keep track
> > of pages in MIN_MTHP_ORDER chunks via a bitmap. Each bit represents a
> > utilized region of order MIN_MTHP_ORDER ptes. We remove the restriction
> > of max_ptes_none during the scan phase so we dont bailout early and miss
> > potential mTHP candidates.
> >
> > After the scan is complete we will perform binary recursion on the
> > bitmap to determine which mTHP size would be most efficient to collapse
> > to. max_ptes_none will be scaled by the attempted collapse order to
> > determine how full a THP must be to be eligible.
> >
> > If a mTHP collapse is attempted, but contains swapped out, or shared
> > pages, we dont perform the collapse.
> >
> > Signed-off-by: Nico Pache <npache@...hat.com>
> > ---
> > mm/khugepaged.c | 122 ++++++++++++++++++++++++++++++++----------------
> > 1 file changed, 83 insertions(+), 39 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index c8048d9ec7fb..cd310989725b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1127,13 +1127,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > {
> > LIST_HEAD(compound_pagelist);
> > pmd_t *pmd, _pmd;
> > - pte_t *pte;
> > + pte_t *pte, mthp_pte;
> > pgtable_t pgtable;
> > struct folio *folio;
> > spinlock_t *pmd_ptl, *pte_ptl;
> > int result = SCAN_FAIL;
> > struct vm_area_struct *vma;
> > struct mmu_notifier_range range;
> > + unsigned long _address = address + offset * PAGE_SIZE;
> > VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> >
> > /*
> > @@ -1148,12 +1149,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > *mmap_locked = false;
> > }
> >
> > - result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
> > + result = alloc_charge_folio(&folio, mm, cc, order);
> > if (result != SCAN_SUCCEED)
> > goto out_nolock;
> >
> > mmap_read_lock(mm);
> > - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> > + *mmap_locked = true;
> > + result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
> > if (result != SCAN_SUCCEED) {
> > mmap_read_unlock(mm);
> > goto out_nolock;
> > @@ -1171,13 +1173,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > * released when it fails. So we jump out_nolock directly in
> > * that case. Continuing to collapse causes inconsistency.
> > */
> > - result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> > - referenced, HPAGE_PMD_ORDER);
> > + result = __collapse_huge_page_swapin(mm, vma, _address, pmd,
> > + referenced, order);
> > if (result != SCAN_SUCCEED)
> > goto out_nolock;
> > }
> >
> > mmap_read_unlock(mm);
> > + *mmap_locked = false;
> > /*
> > * Prevent all access to pagetables with the exception of
> > * gup_fast later handled by the ptep_clear_flush and the VM
> > @@ -1187,7 +1190,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > * mmap_lock.
> > */
> > mmap_write_lock(mm);
> > - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> > + result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
> > if (result != SCAN_SUCCEED)
> > goto out_up_write;
> > /* check if the pmd is still valid */
> > @@ -1198,11 +1201,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > vma_start_write(vma);
> > anon_vma_lock_write(vma->anon_vma);
> >
> > - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
> > - address + HPAGE_PMD_SIZE);
> > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, _address,
> > + _address + (PAGE_SIZE << order));
> > mmu_notifier_invalidate_range_start(&range);
> >
> > pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> > +
> > /*
> > * This removes any huge TLB entry from the CPU so we won't allow
> > * huge and small TLB entries for the same virtual address to
> > @@ -1216,10 +1220,10 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > mmu_notifier_invalidate_range_end(&range);
> > tlb_remove_table_sync_one();
> >
> > - pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> > + pte = pte_offset_map_lock(mm, &_pmd, _address, &pte_ptl);
> > if (pte) {
> > - result = __collapse_huge_page_isolate(vma, address, pte, cc,
> > - &compound_pagelist, HPAGE_PMD_ORDER);
> > + result = __collapse_huge_page_isolate(vma, _address, pte, cc,
> > + &compound_pagelist, order);
> > spin_unlock(pte_ptl);
> > } else {
> > result = SCAN_PMD_NULL;
> > @@ -1248,8 +1252,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > anon_vma_unlock_write(vma->anon_vma);
> >
> > result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> > - vma, address, pte_ptl,
> > - &compound_pagelist, HPAGE_PMD_ORDER);
> > + vma, _address, pte_ptl,
> > + &compound_pagelist, order);
> > pte_unmap(pte);
> > if (unlikely(result != SCAN_SUCCEED))
> > goto out_up_write;
> > @@ -1260,20 +1264,37 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > * write.
> > */
> > __folio_mark_uptodate(folio);
> > - pgtable = pmd_pgtable(_pmd);
> > -
> > - _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> > - _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> > -
> > - spin_lock(pmd_ptl);
> > - BUG_ON(!pmd_none(*pmd));
> > - folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
> > - folio_add_lru_vma(folio, vma);
> > - pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > - set_pmd_at(mm, address, pmd, _pmd);
> > - update_mmu_cache_pmd(vma, address, pmd);
> > - deferred_split_folio(folio, false);
> > - spin_unlock(pmd_ptl);
> > + if (order == HPAGE_PMD_ORDER) {
> > + pgtable = pmd_pgtable(_pmd);
> > + _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> > + _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> > +
> > + spin_lock(pmd_ptl);
> > + BUG_ON(!pmd_none(*pmd));
> > + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> > + folio_add_lru_vma(folio, vma);
> > + pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > + set_pmd_at(mm, address, pmd, _pmd);
> > + update_mmu_cache_pmd(vma, address, pmd);
> > + deferred_split_folio(folio, false);
> > + spin_unlock(pmd_ptl);
> > + } else { //mTHP
> > + mthp_pte = mk_pte(&folio->page, vma->vm_page_prot);
> > + mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
> > +
> > + spin_lock(pmd_ptl);
> > + folio_ref_add(folio, (1 << order) - 1);
> > + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> > + folio_add_lru_vma(folio, vma);
> > + spin_lock(pte_ptl);
> > + set_ptes(vma->vm_mm, _address, pte, mthp_pte, (1 << order));
> > + update_mmu_cache_range(NULL, vma, _address, pte, (1 << order));
> > + spin_unlock(pte_ptl);
> > + smp_wmb(); /* make pte visible before pmd */
> > + pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> > + deferred_split_folio(folio, false);
> > + spin_unlock(pmd_ptl);
> > + }
> >
> > folio = NULL;
> >
> > @@ -1353,21 +1374,27 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > {
> > pmd_t *pmd;
> > pte_t *pte, *_pte;
> > + int i;
> > int result = SCAN_FAIL, referenced = 0;
> > int none_or_zero = 0, shared = 0;
> > struct page *page = NULL;
> > struct folio *folio = NULL;
> > unsigned long _address;
> > + unsigned long enabled_orders;
> > spinlock_t *ptl;
> > int node = NUMA_NO_NODE, unmapped = 0;
> > bool writable = false;
> > -
> > + int chunk_none_count = 0;
> > + int scaled_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - MIN_MTHP_ORDER);
> > + unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0;
> > VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> >
> > result = find_pmd_or_thp_or_none(mm, address, &pmd);
> > if (result != SCAN_SUCCEED)
> > goto out;
> >
> > + bitmap_zero(cc->mthp_bitmap, MAX_MTHP_BITMAP_SIZE);
> > + bitmap_zero(cc->mthp_bitmap_temp, MAX_MTHP_BITMAP_SIZE);
> > memset(cc->node_load, 0, sizeof(cc->node_load));
> > nodes_clear(cc->alloc_nmask);
> > pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> > @@ -1376,8 +1403,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > goto out;
> > }
> >
> > - for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> > - _pte++, _address += PAGE_SIZE) {
> > + for (i = 0; i < HPAGE_PMD_NR; i++) {
> > + if (i % MIN_MTHP_NR == 0)
> > + chunk_none_count = 0;
> > +
> > + _pte = pte + i;
> > + _address = address + i * PAGE_SIZE;
> > pte_t pteval = ptep_get(_pte);
> > if (is_swap_pte(pteval)) {
> > ++unmapped;
> > @@ -1400,16 +1431,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > }
> > }
> > if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > + ++chunk_none_count;
> > ++none_or_zero;
> > - if (!userfaultfd_armed(vma) &&
> > - (!cc->is_khugepaged ||
> > - none_or_zero <= khugepaged_max_ptes_none)) {
> > - continue;
> > - } else {
> > + if (userfaultfd_armed(vma)) {
>
> Its likely you might introduce a regression in khugepaged cpu usage over here.
>
> For intel x86 machines that dont have TLB coalescing (AMD) or contpte (ARM),
> there is a reduced benefit of mTHPs, I feel like a lot of people will never
> change from the current kernel default, i.e. 2M THPs and fallback to 4K.
>
> If you are only parsing 2M hugepages, early bailout when
> none_or_zero <= khugepaged_max_ptes_none is a good optimization, and getting
> rid of that will cause a regression? It might look a bit out of place,
> but is it possible to keep this restriction of max_ptes_none during the
> scanning phase if only PMD mappable THPs are allowed?
That seems like a reasonable request, I will try to implement that. Thanks!
>
>
>
> > result = SCAN_EXCEED_NONE_PTE;
> > count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> > goto out_unmap;
> > }
> > + continue;
> > }
> > if (pte_uffd_wp(pteval)) {
> > /*
> > @@ -1500,7 +1529,16 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > folio_test_referenced(folio) || mmu_notifier_test_young(vma->vm_mm,
> > address)))
> > referenced++;
> > +
> > + /*
> > + * we are reading in MIN_MTHP_NR page chunks. if there are no empty
> > + * pages keep track of it in the bitmap for mTHP collapsing.
> > + */
> > + if (chunk_none_count < scaled_none &&
> > + (i + 1) % MIN_MTHP_NR == 0)
> > + bitmap_set(cc->mthp_bitmap, i / MIN_MTHP_NR, 1);
> > }
> > +
> > if (!writable) {
> > result = SCAN_PAGE_RO;
> > } else if (cc->is_khugepaged &&
> > @@ -1513,10 +1551,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > out_unmap:
> > pte_unmap_unlock(pte, ptl);
> > if (result == SCAN_SUCCEED) {
> > - result = collapse_huge_page(mm, address, referenced,
> > - unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0);
> > - /* collapse_huge_page will return with the mmap_lock released */
> > - *mmap_locked = false;
> > + enabled_orders = thp_vma_allowable_orders(vma, vma->vm_flags,
> > + tva_flags, THP_ORDERS_ALL_ANON);
> > + result = khugepaged_scan_bitmap(mm, address, referenced, unmapped, cc,
> > + mmap_locked, enabled_orders);
> > + if (result > 0)
> > + result = SCAN_SUCCEED;
> > + else
> > + result = SCAN_FAIL;
> > }
> > out:
> > trace_mm_khugepaged_scan_pmd(mm, &folio->page, writable, referenced,
> > @@ -2476,11 +2518,13 @@ static int khugepaged_collapse_single_pmd(unsigned long addr, struct mm_struct *
> > fput(file);
> > if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > mmap_read_lock(mm);
> > + *mmap_locked = true;
> > if (khugepaged_test_exit_or_disable(mm))
> > goto end;
> > result = collapse_pte_mapped_thp(mm, addr,
> > !cc->is_khugepaged);
> > mmap_read_unlock(mm);
> > + *mmap_locked = false;
> > }
> > } else {
> > result = khugepaged_scan_pmd(mm, vma, addr,
>
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